Multiple frequency pulsing of multiple coil source to control plasma ion density radial distribution

ABSTRACT

A method is provided for processing a workpiece supported on a support surface in a chamber of a plasma reactor. A process gas is introduced into the chamber and a plasma is generated with pulse-modulated RF power. The method comprises successively repeating the following cycle: (a) concentrating the plasma in the chamber in a center-high plasma ion distribution for a first on-time duration; (b) permitting plasma to drift during a first off-time duration away from the center-high plasma ion distribution; (c) concentrating the plasma in the chamber in an edge-high plasma ion distribution for a second on-time duration; and (d) permitting plasma to drift during a second off-time duration away from the edge-high plasma ion distribution. The method further comprises adjusting a plasma process rate near a center of the workpiece by adjusting a duty cycle of the first on-time and first off-time. The method also comprises adjusting a plasma process rate near a periphery of the workpiece by adjusting a duty cycle of the second on-time and second off-time.

TECHNICAL FIELD

The disclosure concerns the processing of a workpiece or semiconductorwafer in a plasma reactor having plural overhead coils for applying RFplasma source power, and in particular a method for controlling andimproving uniformity of the radial distribution of plasma ion density.

BACKGROUND

A plasma reactor that generates an inductively coupled plasma is capableof etching thin films on a workpiece such as a semiconductor wafer at arelatively high etch rate. Such a reactor has an inductively coupledplasma source power applicator, typically a coil antenna, coupled to anRF power generator. As the wafer diameter has increased in recent years,the chamber size has increased accordingly, requiring larger coilantennas, which greater inductance and more concentrated powerdeposition profiles. Power deposition tends to peak in narrow annularregions underlying the coil antenna or underlying inner and outer coilantennas. Such concentrated profiles cause large peaks in the plasma iondensity distribution that are difficult to compensate, leading toreduced process uniformity across the wafer. Some improvement in processuniformity can be achieve using two (or more) concentric coil antennasover the reactor ceiling, one antenna overlying the wafer periphery andthe other being closer to the wafer center. Even though such aconfiguration can improve process uniformity, the concentrated peaks inthe power deposition profiles of the inner and outer coil antennas leadto process non-uniformities that are difficult to reduce.

SUMMARY

A method is provided for processing a workpiece supported on a supportsurface in a chamber of a plasma reactor. A process gas is introducedinto the chamber and a plasma is generated with pulse-modulated RFpower. The method comprises successively repeating the following cycle:(a) concentrating the plasma in the chamber in a center-high plasma iondistribution for a first on-time duration; (b) permitting plasma todrift during a first off-time duration away from the center-high plasmaion distribution; (c) concentrating the plasma in the chamber in anedge-high plasma ion distribution for a second on-time duration; and (d)permitting plasma to drift during a second off-time duration away fromthe edge-high plasma ion distribution. The method further comprisesadjusting a plasma process rate near a center of the workpiece byadjusting a duty cycle of the first on-time and first off-time. Themethod also comprises adjusting a plasma process rate near a peripheryof the workpiece by adjusting a duty cycle of the second on-time andsecond off-time.

In one embodiment, the adjustment of the plasma process rate near acenter of the workpiece comprises reducing the plasma process rate nearthe center of the workpiece and the adjusting the first duty cyclecomprises reducing the first duty cycle. In one embodiment, theadjustment of a plasma process rate near a periphery of the workpiececomprises reducing the plasma process rate near the periphery of theworkpiece and the adjusting the second duty cycle comprises reducing thesecond duty cycle.

In one embodiment, the reduction in plasma process rate near the centerof the workpiece and the reduction in plasma process rate near theperiphery of the workpiece reduces non-uniformity in distribution ofprocess rate across the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited embodiments of theinvention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe had by reference to the embodiments thereof which are illustrated inthe appended drawings. It is to be noted, however, that the appendeddrawings illustrate only typical embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

FIG. 1 illustrates a plasma reactor adapted to carry out processesdisclosed herein.

FIGS. 2A, 2B and 2C illustrate a chronological sequence depicting howplasma ion distribution spreads out over time following a trailing edgeof pulsed RF power applied to the inner coil only in the reactor of FIG.1.

FIGS. 3A, 3B and 3C illustrate a chronological sequence depicting howplasma ion distribution spreads out over time following a trailing edgeof pulsed RF power applied to the outer coil only in the reactor of FIG.1.

FIGS. 4A and 4B are contemporaneous timing diagrams of pulse waveformsthat pulse-modulate RF power applied to the inner and outer coils,respectively, of the reactor of FIG. 1.

FIGS. 5A, 5B, 5C, 5D, 5E and 5F illustrate a chronological sequencedepicting how plasma ion distribution alternately (a) concentratesduring pulse on times below one or the other of the inner and outercoils of FIG. 1, and (b) spreads out during off times between pulses.

FIGS. 6A, 6B, 6C, 6D, 6E and 6F depict radial distributions of etch ratecorresponding to FIGS. 5A, 5B, 5C, 5D, 5E and 5F, respectively.

FIG. 7 illustrates a process in accordance with one embodiment.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. The drawings in the figures are all schematic and not toscale.

DETAILED DESCRIPTION

FIG. 1 illustrates a plasma reactor adapted to carry but certainprocesses in accordance with embodiments disclosed herein. The reactorincludes a vacuum chamber 100 enclosed by a cylindrical side wall 105and a disk-shaped ceiling 110. A wafer support pedestal 120 within thechamber 100 has a top insulating layer 121, a conductive base 122, acylindrical side wall 123 and a mesh electrode 124 within the insulatinglayer 121. The top of the insulating layer 121 defines a wafer supportsurface 125 that is separated from the mesh electrode 124 by a very thinportion of the insulating layer 121. A semiconductor wafer 126 can besupported on the wafer support surface 125. A process gas supply 130furnishes process gases to the chamber 100 through gas injectionapparatus 135 of any suitable type, such as individual gas injectors 135a, 135 b. Chamber pressure is controlled by a vacuum pump 140 and by theflow rate of process gases from the injection apparatus 135. The reactorof FIG. 1 further includes an inductively coupled plasma source powerapplicator consisting of an inner coil antenna 150 and an outer coilantenna 155 concentric with the inner coil antenna 150. Each coilantenna 150, 155 may consist of an individual conductor or wirehelically wound in a torus (as shown in FIG. 1), or may be a flatwinding helical winding. In order to enhance inductive coupling of RFpower from the antennas 150, 155 into the chamber 100, the ceiling 110may be formed of an insulating material. The side wall 105 may be metalthat is coupled to RF ground. The inner coil antenna 150 receives RFpower from a first RF power generator 160 through a first pulse gate 162and through a first RF impedance match circuit or element 164. The outercoil antenna 155 receives RF power from a second RF power generator 165through a second pulse gate 167 and through a second RF impedance matchcircuit 169. A programmable controller 170 or other suitable devicecontrols the pulse gates 162, 167. RF plasma bias power from an RFgenerator 180 is coupled to the wafer 126 by applying it to the meshelectrode 124 (or alternatively to the conductive base 122) through anRF impedance match element 182. Optionally, a second power generator 184may be coupled to the mesh electrode 124 (or alternatively to theconductive base 122) through another impedance match element 186. The RFbias generator 180 may be a low frequency generator while the RFgenerator 184 may be a high frequency or very high frequency generator.The two source power generators 160, 165 may be of similar frequencies(e.g., LF or HF) that are sufficiently offset from one another to avoidcoupling between them, for example.

It is our discovery that pulse-modulating the RF power applied to thecoil antennas 150, 155 can be performed or controlled in such a way asto solve the problem non-uniformity caused by the concentrated powerdeposition profile of the coil antennas 150, 155. In one embodiment,this is accomplished by causing the plasma to alternate betweendifferent ion density distributions, so that the plasma processingresults reflect a time average of the different distributions. Duringeach pulse cycle, RF power is turned off at the trailing edge of thepulse, which permits the plasma to drift away from a concentratedprofile to a more diffuse profile. This movement in plasma distributionprovides an time-averaged plasma distribution that has betteruniformity.

In one embodiment, FIGS. 2A, 2B and 2C depict plasma distribution acrossthe process region overlying the wafer 126 in chronological sequence.The regions labeled 190 and 191 in each of FIGS. 2A, 2B and 2Ccorrespond to zones of concentrated plasma ion density. For example, theregion 190 may have on the average of about 10¹¹ ions/cc, while theregion 191 may have on the average of about 1.5·10¹⁰ ions/cc. Theremainder of the chamber 100 outside of both regions 190, 191 has a muchlower plasma ion density, on the average less than 10¹⁰ ions/cc. FIG. 2Arepresents the distribution during the time that RF power is applied tothe inner coil antenna 150 only. FIG. 2B illustrates the distributionshortly after power has been turned off and FIG. 2C illustrates thedistribution after power has been turned off for a somewhat longer time.The time differences between FIGS. 2A, 2B and 2C may be on the order of0.01-100 milliseconds. In FIG. 2A, plasma ions concentrate over thecenter of the wafer. In FIG. 2B, removal of RF power causes the plasmato drift out of the concentrated profile of FIG. 2A and begin todistribute outwardly away from the center. In FIG. 2C, the continuedrift of the plasma results in any even greater radial spreading ofplasma ion distribution away from the center and toward the periphery ofthe process region.

In one mode, the inner coil RF power is pulse modulated by the gate 162with a desired repetition rate and duty cycle in which the plasma iondistribution corresponds to FIG. 2A during the “ON” time and during thetime between pulses from the distribution of FIG. 2A to that of FIG. 2Band later to that of FIG. 2C. The spreading of the distribution depictedin FIGS. 2B and 2C is halted at the beginning of the next cycle when RFpower is again applied to the inner coil 150.

In another embodiment, FIGS. 3A, 3B and 3C depict plasma distributionacross the process region overlying the wafer 126 in anotherchronological sequence involving the outer coil 155. The regions labeled193 and 194 in each of FIGS. 3A, 3B and 3C correspond to zones ofconcentrated plasma ion density. For example, the region 193 may have onthe average of about 10¹¹ ions/cc, while the region 194 may have on theaverage of about 1.5.10¹⁰ ions/cc. The remainder of the chamber 100outside of both regions 193, 194 has a much lower plasma ion density, onthe average less than 10¹⁰ ions/cc. FIG. 3A represents the distributionduring the time that RF power is applied to the outer coil antenna 155only. FIG. 3B illustrates the distribution shortly after power has beenturned off and FIG. 3C illustrates the distribution after power has beenturned off for a somewhat longer time. The time differences betweenFIGS. 3A, 3B and 3C may be on the order of 0.01-100 milliseconds. InFIG. 3A, plasma ions concentrate over or near the periphery of thewafer. In FIG. 3B, removal of RF power causes the plasma to drift out ofthe concentrated profile of FIG. 3A and begin to distribute inwardlytoward the center and away from the periphery. In FIG. 3C, the continuedrift of the plasma results in any even greater radial spreading ofplasma ion distribution away from the periphery and toward the center ofthe process region.

In one mode, the outer coil RF power is pulse modulated by the gate 167with a desired repetition rate and duty cycle in which the plasma iondistribution corresponds to FIG. 3A during the “ON” time and during thetime between pulses from the distribution of FIG. 3A to that of FIG. 3Band later to that of FIG. 3C. The spreading of the distribution depictedin FIGS. 3B and 3C is halted at the beginning of the next cycle when RFpower is again applied to the outer coil 155.

In one mode, pulses of RF power are applied alternately to the inner andouter coils 150, 155. FIGS. 4A and 4B are contemporaneous time domaindiagrams of enabling signals applied to the pulse gates 162, 167 by thecontroller 170 in such a mode. FIGS. 5A through 5F depict achronological sequence of changing plasma ion distributions in theprocess zone over one cycle corresponding to FIGS. 4A and 4B. Referenceis now made to FIGS. 4A, 4B and 5A through 5F. From time T1 to time T2,RF power is applied to the inner coil 150 only, so that plasmadistribution is concentrated over the center of the process region (FIG.5A). At time T2, power is turned off, and the plasma begins to drift orprogressively spread outward and away from the center to become lessconcentrated. This trend (FIGS. 5B and 5C) continues until time T3, whenRF power is applied to the outer coil 155. The causes the plasma toconcentrate near the edge of the process zone (FIG. 5D). At time T4,power is turned off, and the plasma begins to drift toward the center ofthe process region, spreading more over time as depicted in FIGS. 5E and5F. This continues until time T5, when a new cycle is begun and power isagain applied to the inner coil 150, at which point the distributionreturns to that depicted in FIG. 5A.

The resulting change in etch rate distribution is depicted in thechronological sequence of FIGS. 6A through 6F. In FIG. 6A, etch ratepeaks at the wafer center (time T1 through time T2) during applicationof RF power to the inner coil 150. In FIGS. 6B and 6C, etch rate beginsto decrease over the center and increase away from the center (time T2through T3). In FIG. 6D, progression of etch rate away from the centerand toward the edge results in concentration of the maximum etch rate atthe periphery (time T3 through T4) while RF power is applied to theouter coil 155. In FIGS. 6E and 6F, RF power is turned off and etch ratedistribution drifts back away from the periphery and toward the center.The foregoing cycle repeats itself at time T5.

Etch results on the wafer at the end of the etch process are thetime-average of all of the etch rate distributions (samples of which aredepicted in FIGS. 6A through 6F) over the entire etch process. Theprogression of etch rate distribution actually consists of a continuumof distributions, not merely the six discrete distributions of FIGS. 6Athrough 6F. The time average spans this continuum of distributions. Thistime average of etch rate (or ion density) distribution is far moreuniform than can be achieved by the conventional methods in which RFpower is continuously applied to one or both of the inner and outercoils 150, 155. The plasma drift resulting in the progression of etchrate distribution as depicted in FIGS. 6A-6F provides a far morecontinuous movement of etch rate across the wafer, which tends to reduceor eliminate areas of lower etch rate or areas of peak etch rate.

In an exemplary process, a polysilicon film overlying a thin gate oxidelayer is to be etched to form polysilicon gates. A silicon etch gas,such a fluorine-containing species, is introduced into the chamber 100of FIG. 1 with the wafer 126 supported on the pedestal 120. RF biaspower is applied to the electrode 124 from the RF generator 180. Thecontroller 170 causes RF power to be applied alternately to the innerand outer coils 150, 155 in accordance with the waveforms of FIGS. 4Aand 4B. The duty cycles of the pulses applied to the inner coil gate 162(FIG. 4A) and of the pulses applied to the outer coil gate 167 (FIG. 4B)are adjusted to maximize uniformity of the time average of theprogression of etch rate distributions realized over many cycles of thewaveforms of FIGS. 4A and 4B.

For example, if the etch rate distribution is too concentrated at thecenter during the inner coil on-time (times T1-T2) and moreover is tooconcentrated at the edge during the outer coil on-time (times T3-T4),then this excessive concentration is compensated by reducing the dutycycles of both the pulses applied to the inner coil gate 162 (FIG. 4A)and the pulses applied to the outer coil gate 167 (FIG. 4B). This allowsgreater time during which no RF power is applied to either coil 150, 155and the plasma drifts away from its more concentrated distributionstates, and provides a more uniform time-averaged etch ratedistribution. The duty cycles of the control pulses governing the pulsedRF on the two coils 150, 155 (FIGS. 4A and 4B) may be the same or may bedifferent depending upon the differences in design or performance of thetwo coils 150, 155. In the illustrated example of FIGS. 4A and 4B, theduty cycles of the pulse waveform of FIG. 4A and the pulse waveform ofFIG. 4B are approximately the same and are on the order of about ⅙.However, other choices of duty cycle may be made depending upon aparticular reactor design and process recipe. In another embodiment, theduty cycle may be increased, so as to decrease the power off interval(e.g., from time T2 to time T3) to a lesser time period. In the exampleof FIGS. 4A and 4B, the pulse widths of the two control signals aredepicted as being the same. However, the pulse widths (duty cycles) ofthe pulse signals of FIGS. 4A and 4B may be chosen independently anddiffer significantly from one another.

As one example involving different duty cycles applied to the inner andouter coils 150, 155, if etch rate is higher over the center and weakerat the periphery, then the duty cycle of the pulse waveform of FIG. 4Aapplied to the inner coil gate 162 may be decreased and/or the dutycycle of the pulse waveform of FIG. 4B applied to the outer coil gate167 may be increased. Conversely, if etch rate is predominant over theperiphery and weak at the center, then the duty cycle of the pulsewaveform of FIG. 4A applied to the inner coil gate 162 may be increasedand/or the duty cycle of the pulse waveform of FIG. 4B applied to theouter coil gate 167 may be decreased.

In some applications, the duty cycles of the pulsing of the gates 162,167 may be set to relatively high values. For example, both duty cyclesmay exceed 50%, in which case the “on” time periods of the two coils150, 155 will be partially contemporaneous or overlapping. In otherwords, each coil will be turned off after the other coil has been turnedon. In this case, RF coupling between the two coils can be minimized byoffsetting the frequencies of the two RF generators 160, 165. As onepossible example of this, the two RF generators may have respectivefrequencies of 2.75 MHz and 2.25 MHz.

The period or length of one cycle in the pulse waveforms of FIGS. 4A and4B (i.e., the period from time T1 to time T5) is in one embodimentrelatively short, for example on the order of 0.01-100 milliseconds.Shorter values of this time period provide the best continuity of etchperformance and minimize fluctuations within a single etch process orstep.

A process in accordance with one embodiment is depicted in FIG. 7. Thewafer 126 is placed in the chamber 100 and a process gas is introduced(block 200 of FIG. 7). RF power is applied to the inner and outer coils150, 155 through the respective pulse gates 162, 167 (block 210). Thecontroller 170 enables power flow through the respective gates 162, 167during alternate time windows defined by applying alternating pulseswaveforms to the gates 162, 167 (block 215). The duty cycles of therespective pulse waveforms are adjusted to optimize uniformity of radialion distribution over the wafer (block 220). If the inner coil 150produces an excessively concentrated or peak ion distribution or etchrate over the wafer center, then the duty cycle of the pulse waveformapplied to the inner coil 150 is reduced (block 221). This decrease ininner coil duty cycle allows more time for plasma drift followingtemporary RF power removal to spread or even out the ion distribution orcounter the non-uniform distribution created during the preceding pulseof RF power. Alternatively (or in addition) to decreasing the inner coilduty cycle, the outer coil duty cycle may be increased, provided thatthis increase does not result in excessive ion concentration by theouter coil 155. If the outer coil 155 produces an excessivelyconcentrated or peak ion distribution or etch rate over the waferperiphery, then the duty cycle of the pulse waveform applied to theouter coil 155 is reduced (block 222). This decrease in outer coil dutycycle allows more time for plasma drift following temporary RF powerremoval to spread or even out the ion distribution or counter thenon-uniform distribution created during the preceding pulse of RF power.Alternatively (or in addition) to decreasing the outer coil duty cycle,the inner coil duty cycle may be increased provided that this increasedoes not result in excessive ion concentration by the inner coil 150. Inone embodiment, if a higher overall plasma ion density (or higherprocess rate or higher etch rate) is desired, then the duty cycle of oneor both waveforms may be increased up to a point at which uniformity maybe compromised (block 223).

While foregoing embodiments have been described with reference to RFgenerators 160, 165 with pulsed gates 162, 167 for pulse modulating theRF outputs of the generators 160, 165, the generator 160 andcorresponding gate 162 may be combined in one unit as a commerciallyavailable pulse-modulated RF generator. Likewise, the generator 165 andcorresponding gate 167 may be combined in another similar unit.

While the foregoing description of embodiments having at least two coils(e.g., the inner and outer coils 150, 155) have been described withreference to operational modes in which the RF power to both coils ispulse-modulated, in another embodiment both coils 150, 155 are drivenwith RF power but only one of the two coils is driven withpulse-modulated RF power. In such an embodiment, for example, RF powerto the inner coil 150 would be pulse modulated in accordance with thesequence of FIG. 4A, while RF power to the outer coil would be appliedcontinuously. Alternatively, RF power to the outer coil 155 would bepulsed modulated in accordance with the sequence of FIG. 4B, while RFpower to the inner coil would be applied continuously. The controller170 may be configured to implement either of these embodiments bypulsing one of the two gates 162, 167 while continuously enabling(holding “on”) the other of the two gates. This corresponds to anoperational mode in which the pulse duty cycle to one of the coils is100%.

While the foregoing description of embodiments having at least two coils(e.g., the inner and outer coils 150, 155) have been described withreference to separate independent RF power generators for each coil(e.g., the RF power generators 160, 165), in another embodiment only asingle RF generator is employed and has its RF output power apportionedamong the different coils. For example, as indicated in dashed line inFIG. 1, the RF generator 160 may have its output coupled to both gates162, 167. In this case, the individual gates 162, 167 perform thepulse-modulation functions governed by the controller 170 as describedabove, and in addition each includes conventional RF circuitry thatenables the controller 170 to control the amount of RF power admittedthrough each of the gates 162, 167. With this latter feature, thecontroller 170 in this embodiment apportions the RF power from thegenerator 160 to the two coils 150, 155.

While foregoing embodiments have been described with reference to aninductively coupled RF power applicator consisting of two coils, aninner coil 150 and an outer coil 155, in another embodiment there may beonly a single coil (e.g., either the inner coil 150 or the outer coil155 or a single coil at an intermediate location). Alternatively, morethan one coil may be present, but only a single coil is driven by RFpower, the remaining coil (or coils) being inactive. The single coilwould be driven by a single RF power generator (e.g., the generator 160)with pulse modulation of the RF power being performed by a pulsed gate(e.g., the gate 162) pulsed by the controller 170 in accordance with achosen pulsing sequence.

While embodiments having more than one coil have been described abovewith reference to two coils (i.e., the inner and outer coils 160, 165),such embodiments may have more than two coils, e.g., three or four ormore coils. Generally at least some or all of such coils may beconcentric as in the embodiment of FIG. 1.

While the foregoing is directed to embodiments of the invention, otherand further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method of processing a workpiece supported on a support surface ina chamber of a plasma reactor, comprising: supplying a process gas intosaid chamber, said chamber comprising a concentric inner and outer coilantennas over the chamber and facing the support surface and saidchamber comprises a first and second pulse-modulated RF power sources offirst and second RF frequencies for respective ones of said inner andouter coil antennas; applying pulse-modulated RF power to said inner andouter coil antennas by successively repeating the following cycle: (a)applying RF power from said first source to said inner coil antenna fora first on-time duration corresponding to a first duty cycle, and at theend of said first on-time duration refraining from applying RF power tosaid inner coil antenna; (b) applying RF power from said second sourceto said outer coil antenna for a second on-time duration correspondingto a second duty cycle, and at the end of said second on-time durationrefraining from applying RF power to said outer coil antenna; adjustinga plasma process rate near a center of said workpiece by adjusting saidfirst duty cycle; and adjusting a plasma process rate near a peripheryof said workpiece by adjusting said second duty cycle.
 2. The method ofclaim 1 wherein adjusting the plasma process rate near the center ofsaid workpiece comprises reducing the plasma process rate near thecenter of the workpiece and said adjusting said first duty cyclecomprises reducing said first duty cycle.
 3. The method of claim 2wherein adjusting the plasma process rate near the periphery of saidworkpiece comprises reducing the plasma process rate near the peripheryof the workpiece and said adjusting said second duty cycle comprisesreducing said second duty cycle.
 4. The method of claim 3 wherein saidreducing the plasma process rate near said center of said workpiece andsaid reducing of the plasma process rate near said periphery of saidworkpiece reduces non-uniformity in distribution of process rate acrosssaid workpiece.
 5. The method of claim 1 wherein said first and secondfrequencies are offset from one another.
 6. The method of claim 1further comprising coupling RF bias power to said workpiece.
 7. Themethod of claim 6 wherein said RF bias power has a frequency differentfrom said first and second frequencies.
 8. The method of claim 1 whereinsaid process gas comprises an etchant precursor and the plasma processrate an etch rate.
 9. The method of claim 1 wherein said cycle has aperiod of about 0.01-100 milliseconds.
 10. The method of claim 1 whereinthe durations of said first and second on-time durations are one theorder to 0.01-100 milliseconds.
 11. A method of processing a workpiecesupported on a support surface in a chamber of a plasma reactor,comprising: introducing a process gas into the chamber and generating aplasma in said chamber with pulse-modulated RF power; successivelyrepeating the following cycle: (a) concentrating the plasma in saidchamber in a center-high plasma ion distribution for a first on-timeduration; (b) permitting plasma to drift during a first off-timeduration away from said center-high plasma ion distribution; (c)concentrating the plasma in said chamber in an edge-high plasma iondistribution for a second on-time duration; (d) permitting plasma todrift during a second off-time duration away from said edge-high plasmaion distribution; adjusting a plasma process rate near a center of saidworkpiece by adjusting a duty cycle of said first on-time and firstoff-time; and adjusting a plasma process rate near a periphery of saidworkpiece by adjusting a duty cycle of said second on-time and secondoff-time.
 12. The method of claim 11 wherein adjusting the plasmaprocess rate near the center of said workpiece comprises reducing theplasma process rate near the center of the workpiece and said adjustingsaid first duty cycle comprises reducing said first duty cycle.
 13. Themethod of claim 12 wherein adjusting the plasma process rate near theperiphery of said workpiece comprises reducing the plasma process ratenear the periphery of the workpiece and said adjusting said second dutycycle comprises reducing said second duty cycle.
 14. The method of claim13 wherein said reducing the plasma process rate near said center ofsaid workpiece and said reducing of the plasma process rate near saidperiphery of said workpiece reduces non-uniformity in distribution ofprocess rate across said workpiece.
 15. The method of claim 11 whereinsaid first and second frequencies are offset from one another.
 16. Themethod of claim 11 further comprising coupling RF bias power to saidworkpiece.
 17. The method of claim 16 wherein said RF bias power has afrequency different from said first and second frequencies.
 18. Themethod of claim 11 wherein said process gas comprises an etchantprecursor and the plasma process rate an etch rate.
 19. The method ofclaim 11 wherein said cycle has a period of about 0.01-100 milliseconds.20. The method of claim 11 wherein the durations of said first andsecond on-time durations are one the order to 0.01-100 milliseconds. 21.The method of claim 1 or 11 further comprising holding one of said firstand second duty cycles at 100%.
 22. A method of processing a workpiecesupported on a support surface in a chamber of a plasma reactor,comprising: supplying a process gas into said chamber, said chambercomprising concentric inner and outer coil antennas over the chamber andfacing the support surface; continuously applying RF power to one ofsaid inner and outer coil antennas; applying pulse-modulated RF power tothe other one of said inner and outer coil antennas by successivelyrepeating the following cycle: (a) applying RF power to said other coilantenna for an on-time duration corresponding to a duty cycle, (b) atthe end of said on-time duration refraining from applying RF power tosaid other coil antenna; and adjusting radial distribution of a plasmaprocess rate over said workpiece by adjusting said duty cycle.
 23. Amethod of processing a workpiece supported on a support surface in achamber of a plasma reactor, comprising: supplying a process gas intosaid chamber, said chamber comprising concentric inner and outer coilantennas over the chamber and facing the support surface, and saidchamber configured to receive RF power provided from a common RF powersource to said inner and outer coil antennas; apportioning RF power fromsaid common RF power source to said inner and outer coil antennas;pulse-modulating the RF power applied to said inner and outer coilantennas by successively repeating the following cycle of (a) followedby (b): (a) applying RF power from said common source to said inner coilantenna for a first on-time duration corresponding to a first dutycycle, and at the end of said first on-time duration refraining fromapplying RF power to said inner coil antenna; (b) applying RF power fromsaid common source to said outer coil antenna for a second on-timeduration corresponding to a second duty cycle, and at the end of saidsecond on-time duration refraining from applying RF power to said outercoil; adjusting a plasma process rate near a center of said workpiece byadjusting said first duty cycle; and adjusting a plasma process ratenear a periphery of said workpiece by adjusting said second duty cycle.24. A method of processing a workpiece supported on a support surface ina chamber of a plasma reactor, comprising: supplying a process gas intosaid chamber, said chamber comprising a coil antenna; applyingpulse-modulated RF power to said coil antenna by successively repeatingthe following cycle: (a) applying RF power to said coil antenna for afirst on-time duration corresponding to a duty cycle, (b) at the end ofsaid on-time duration refraining from applying RF power to said coilantenna; and adjusting radial distribution of a plasma process rate byadjusting said duty cycle.